In lasers, an endeavor is made to have a zero-order mode, as is described in German Laid Open Specification No. 3,422,525. The maintenance of an optimal mode is essentially dependent upon whether it is possible to maintain the geometric shape of the laser in operation, because the geometric shape of the laser also determines the shape of the resonator. An essential problem involved in maintaining the geometric shape resides in that, in operation, the laser becomes hot, expands--possibly irregularly--the resonator is thereby given a different geometric shape and thus the mode deteriorates. Accordingly, from the mechanical side it has since been attempted to use low-expansion steels, as they have become known, for example, as Invar steels (trademark of Creusot-Loire (see Rompps Chemie-lexikon [Rompps Chemical Dictionary] 8th edition, page 1906 or Enzyklopadie Naturwissenschaft und Technik [Encyclopedia of Natural Science and Technology], page 2051).
In the Federal Republic of Germany, the applicant previously has demonstrably used a laser which exhibits a continuous support tube for intermediate flanges and end flanges. The end flanges and intermediate flanges are rigidly secured on this support tube, while the glass tubes in which lasing takes place are made of quartz.
In this case, the support tube was made of such an Invar steel. However, this material has the disadvantage that it is extremely expensive and exhibits quite substantially poorer processing qualities than normal steel.
These lasers have, at their end flanges, 180 degree mirrors, one of which is non-transmitting and the other of which is partially-transmitting. The non-transmitting mirror throws the beam back again into the resonator, while the partially-transmitting mirror couples out the laser beam according to the measure of its transmissivity, but likewise throws the other component back again into the resonator.
For the adjustment of the laser, the mirrors are movable insofar as they are rigidly connected to support plates, the position of which is adjustable by screws--differential screws--in two mutually perpendicular directions. When the laser is hot under operational conditions, the support plate belonging to the non-transmitting and/or paritally-transmitting mirror is set to resonator oscillation conditions which are as close as possible to the zero mode or even reach it.
These screws are made of brass. Brass has, per se, the advantage that the screws can be produced more cheaply, and the accuracy of these screws can also be maintained better when the material is brass.
However, it has emerged that customers attempt to adjust these brass screws, probably because the quality of the oscillation was no longer correct. However, it is more than difficult for a layman to set the mirrors correctly.